Many RF circuits and systems employ inductive components such as inductors and transformers. A few examples of these circuits are: voltage-controlled oscillators, low-noise amplifiers, matching networks, power combiners and power amplifiers.
Integrated inductors and transformers are known to have low quality (given by figures of merit such as the quality factor or Q, and the self-resonance frequency) due to semiconductor process limitations. Still, the limited quality of integrated passives can be tolerated in low-power applications. In high-power applications (for example, a base station), the use of such integrated passives is very rarely employed in the high-power circuitry due to their inefficiency.
Another approach to design inductors and transformers is by using standard bond wire fabrication techniques, the same techniques used for interconnections and package leads. Bond wire inductors and transformers avoid many of the technological limitations of standard integrated planar technologies. For example, they have reduced resistive and substrate losses (and thus, increased quality factors), reduced capacitive parasitics (and hence increased self-resonance frequencies), and can handle substantially higher currents.
One of the major challenges and concerns for the use of inductive components fabricated from bond wires is the relatively low predictability of their specific values, due to magnetic interactions with the surrounding media and manufacturing shape variations. Experimentation can lead to a bond wire shape that yields the expected performance and this shape can be easily replicated, and the use of automatic wire bonding machines with loop control capability can control the manufacturing yield and tolerance of a designed bond wire shape.
A common problem with state-of-the-art bond wire transformers is the complexity of arranging the many series connections required to fabricate the primary and secondary. The series connections are required to achieve the desired turns ratio, each loop of bond wire representing a turn. It can be inefficient in terms of space to provide sufficient bond pads for each series connection, and making each of the connections can be costly. Additionally, such series connection of the loops will increase the resistive losses of the structures, leading to lower power efficiency.